1. Field of the Invention
The present invention relates generally to windshield wiper systems and, more particularly, to a windshield wiper system that utilizes individual direct drive motors for coordinated, but mechanically independent, control of the windshield wipers.
2. Description of the Related Art
Windshield wiper systems commonly employed in the related art include pivotally mounted wiper blades that are oscillated across a windshield between an in-wipe position, typically located near the cowl of an automotive vehicle, and an out-wipe position, usually associated with an A-pillar on the vehicle, in the case of the driver side wiper blade in this representative example. It is typically desirable to maximize the angular velocity of the blade assemblies between the in-wipe and out-wipe positions where the blade assembly is moving across the windshield in front of the driver to reduce the duration of each wipe cycle. On the other hand, it is also desirable to limit noise and inertia loading by reducing the velocity of the blade assemblies as they approach the wipe limits. These are two competing objectives that must be balanced in order to be successfully and economically obtained.
One long-standing design approach that has been employed in the related art includes the use of a single motor assembly, driven in one rotational direction, driving two separate wiper arms across the windshield of a vehicle. This approach requires a fairly complex linkage system to convert the singular angular motion of the wiper motor into the two-way linear reciprocal motion to drive both wiper arms. In the dashboard-firewall area, where these systems are typically installed, this mechanical linkage requires a large amount of underhood space. Moreover, the area near this moving linkage must be kept clear of wires and other vehicle components. Additionally, the moving linkage, with its several pivot and rotational points is subject to mechanical inaccuracies and wear, readily introducing excessive wiper movement.
Nevertheless, for many years, designers and manufacturers were reluctant to depart from this established approach. However, improved vehicle aerodynamics that have fostered vehicle designs having longer sloped front surfaces are leading to windshield designs with more pronounced rake angles that result in larger window surfaces. A wiper system for such windshields must therefore include longer, more massive wiper arms and blades to wipe the required percentage of the larger surface. This has created a number of problems. Most notably, the larger arms and swept surface area increases the size of conventional wiper systems to such an extent that it becomes difficult to fit a single motor system within the typically allotted underhood space. This problem is further aggravated by the same aerodynamic sloped front surfaces of the newer vehicle designs, which reduce the available underhood space. Additionally, the larger area to be swept by the wiper system requires more power and control over the wiper arm that can be provided by a linkage type system.
In response to the changes in vehicle front face design and the loss of available underhood space, the dual motor wiper system has evolved. Representative examples of such systems can be found in U.S. Pat. No. 4,585,980 to Gille et al.; U.S. Pat. No. 4,665,488 to Graham et al.; U.S. Pat. No. 4,900,995 to Wainwright; and U.S. Pat. No. 5,252,897 to Porter et al. These wiper systems are generally directly driven. Additionally, U.S. Pat. No. 5,355,061 to Forham employs a brushless dc motor to operate a direct drive windshield wiper system, as do others that follow. The more recent direct drive wiper blade systems employing dual motors have utilized some hardware and/or software controlled switching scheme to control each individual motor, in reference to the other, to provide blade control across the windshield and prevent blade-to-blade contact.
The conventional control approach relies upon intricate software control and position sensing along the wipe pattern. This undesirably requires separate motor control circuitry and a reliance on the movement of the wiper motors to provide positional feedback. Generally, motor position feedback has been used in brushless dc motors by sensing the changes in the commutation of the motor windings. This has sometimes been done using Hall Effect sensors, as disclosed, for example, in U.S. Pat. No. 4,680,515 to Crook; U.S. Pat. No. 4,723,100 to Horikawa et al.; and U.S. Pat. No. 4,897,583 to Rees. The Hall Effect sensors have also been used to count pulses of a pulse train generated by a rotating toothed wheel to produce position signals for operational control of the motor. While suitable for use in windshield wiper systems, the use of the above-noted brushless dc motor controllers in a windshield wiper system that uses separate position sensors for coordination of the wipers can result in an unnecessarily complicated design. Also, any loss of power to the system will disorient and confuse these sensors such that the wiper arm position becomes an unknown. Thus, windshield wiper systems that employ pulse train type sensors suffer from the disadvantage that they easily loose the accurate position of the windshield wiper blade during common operating conditions and therefore suffer a loss of control in these circumstances.
The build-up of snow and ice on the windshield complicates the control of blade movement and the ability to accurately determine wiper arm position and can impede the movement of the blades unevenly, causing one blade to move faster than the other. When encountering this problem, electronically controlled wiper systems presently known in the art can often become unsynchronized and may clash as they become unable to maintain their sense of wiper arm position. Thus, there is a need in the art for a direct drive motor for a windshield wiper system that has integrated control circuitry and achieves position sensing such that the wiper arms position is known regardless of rotation and such that the detected arm position is not lost during power loss or loss of motion.
Conventional dual direct drive wiper systems use high-speed dc motors. This is undesirable, as it requires large counter-rotational forces to stop and then reverse the wiper arm at the end of its sweep. Also, large current draws are necessary to produce the counter-rotational forces which causes repetitive surges in the supplied power and induces great amounts of electro-magnetic interference to the immediately surrounding parts of the vehicle. With a high-speed dc motor, it is also problematic to vary the speed of the wiper arm as it sweeps across the windshield, if this is desired as part of a sweeping pattern or predetermined clearing scheme. These drawbacks stem from the conventional construction of direct drive wiper motors, which have either a one-to-one direct drive or an inefficient gearing assembly to differ the wiper arm speed from motor speed. Thus, there is also a need in the art for a direct drive motor for a windshield wiper system that is efficient and controllable at a lower drive speed and that is electro-magnetically clean.
One other drawback to conventional wiper motor systems has recently emerged. The conventional direct drive windshield wiper systems employ dc motors that are of the standard 12-volt operating standard. This is presently adequate, but current design trends are moving toward more efficient 42 volt based automotive electrical systems. The change over to a 42 volt automotive electrical systems will be highly problematic for the prior dual direct drive wiper systems and presents a considerable drawback as the prior systems are not compatible. Therefore, there is a need to not only provide a direct drive windshield wiper system that overcomes the above-mentioned drawbacks but that also has the ability to be employed in the newly emerging 42 volt automotive electrical system environment.